Now [when Osgiliath was taken] the Chieftain of the Ringwraiths dwelt in Minas Morgul with six companions, while the second to the Chief, Khamûl the Shadow of the East, abode in Dol Guldur as Sauron's lieutenant, with one other 1 as his messenger.

Unfinished Tales, Part 3, Ch 4, The Hunt for the Ring: Of the Journey of the Black Riders

1According to the entry in the Tale of Years for 2951 Sauron sent three, not two, of the Nazgûl to reoccupy Dol Guldur. The two statements can be reconciled on the assumption that one of the Ringwraiths of Dol Guldur returned afterwards to Minas Morgul but I think it more likely that the formulation of the present text was superseded when the Tale of Years was compiled; and it may be noted that in a rejected version of the present passage there was only one Nazgûl in Dol Guldur (not named as Khamûl, but referred to as "the Second Chief (the Black Easterling)"), while one remained with Sauron as his chief messenger.